Nickel-base superalloy

ABSTRACT

A cast, precipitation-hardening alloy, applicable to turbine blades and vanes operating in the 1,200*-1,900* F. temperature range, of a nominal composition, by weight, of 9.5 percent chromium, 10 percent cobalt, 2.5 percent molybdenum, 3.7 percent tungsten, 4.2 percent titanium, 5.2 percent aluminum, 1 percent vanadium, 0.015 percent boron, 0.06 percent zirconium, balance nickel. The invention herein described was made in the course of or under a contract with the Department of the Air Force.

[111 3,785,809 1451 Jan. 15, 1974 1 NICKEL-BASE SUPERALLOY [75]Inventor: Arthur R. Cox, Lake Park, Fla.

[73] Assignee: United Aircraft Corporation, East Hartford, Conn.

1 Filed! June 15, 1971 Appl. No.: 153,444

[52] US. Cl. 75/171, 148/325 [51] Int. Cl. .r C22C 19/00 [58] Field ofSearch 75/171, 170; 148/32, 148/325 [56] References Cited UNITED STATESPATENTS 3,061,426 10/1962 Bieber 75/171 3,642,469 2/1972 Ross et a1.75/171 3,415,641 12/1968 Ross 75/171 5/1968 MacFarlane et a1 75/1713,260,505 7/1966 Ver Snyder 75/171 Primary ExaminerRichard 0. DeanAtt0rneyRichard N. James [5 7 ABSTRACT A cast, precipitation-hardeningalloy, applicable to turbine blades and vanes operating in thel,200-l,900 F. temperature range, of a nominal composition, by weight,of 9.5 percent chromium, 10 percent cobalt, 2.5 percent molybdenum, 3.7percent tungsten, 4.2 percent titanium, 5.2 percent aluminum, 1 percentvanadium, 0.015 percent boron, 0.06 percent zirconium, balance nickel.

The invention herein described was made in the course of or under acontract with the Department of the Air Force.

2 Claims, No Drawings 1 NICKEL-BASE SUPERALLOY BACKGROUND OF THEINVENTION The present invention relates in general to the cast,precipitation-hardening nickel-base superalloys and, in particular, tosuch alloys having application to gas turbine engine blades and vanes.

One of the advanced nickel-base superalloys which is utilized in gasturbine engine blading is that described in the patent to Bieber3,061,426. This alloy, hereinafter identified as Alloy A, has a nominalcomposition comprising, in percent by weight, chromium, cobalt, 3molybdenum, 4.5 titanium, 5.5 aluminum, 1 vanadium, 0.17 carbon, 0.015boron, 0.06 zirconium, balance nickel.

To meet the ever-increasing demands of the engine designer furtherstrength increases in these alloys are required. In particular,increased creep resistance is of fundamental importance. However,despite the knowledge that the addition of patent strengthening elementssuch as tungsten would possibly impart the desired improvements, it hasbeen found that the above alloy, as constituted. could not be enrichedin such elements because the composition would be forced into sigmaformation and a consequent unacceptable embrittlement.

SUMMARY OF THE INVENTION The present invention contemplates an alloycomprising, in percent by weight, 9-10 chromium, 9.5l0.5 cobalt, 0-5molybdenum, 1-5 tungsten, 3.5-5.5 titanium, 3-7 aluminum, 0.5-l.5vanadium, 0.005-0.025 boron, 0.03-0.l zirconium, 0-0.25 carbon, balanceessentially nickel.

A particular preferred embodiment, hereinafter referred to as FA-375,consists essentially of, in percent by weight, 9-10 chromium, 9.5-l0.5cobalt, 2-3 molybdenum, 3.5-4 tungsten, 4-4.5 titanium, 5-5.5 aluminum,0.7-1 .2 vanadium, 0.015-0.025 boron, 0.04-0.08 zirconium, 0-0.20 carbon(0.15-0.20 for polycrystalline fonn), balance essentially nickel. Inaddition, the following maximums are maintained, 0.50 iron, 0.20manganese, 0.01 phosphorous, 0.01 sulfur and 0.2 silicon. A small amountof hafnium, up to 2.5 weight percent, may optionally be added to thealloy.

In addition to polycrystalline castings, the alloy is adapted todirectional solidification techniques, including those suggested byVerSnyder U.S. Pat. No.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In alloys of the general typedescribed, the nickel component is used to set the particular crystallattice, austenite type, or face-centered cubic system from Indirectional casting and the production of columnar-grain articles, theslow solidification rates associated therewith have revealed, in someinstances, evidence of precracked MC-type carbides which may act asfailure sites in fatigue. Accordingly, in such processes low carboncontents are preferred. I-Iafnium, an expensive rare-earth element, hasbeen shown to provide some improvements in alloy castability and hasgiven evidence of improved creep-rupture ductility.

The provision of an alloy with improved creep strength is not achievedmerely by the addition of known strengthening elements to a given alloyformation. As previously described, Alloy A to commercial specificationchemistry has proven intolerant of the addition of further strengtheningelements because such enrichment has led to the formation of thedetrimental sigma phase.

The sigma phase is usually identified as an intermetallic compound oftopologically close-packed morphology. It precipitates, often in usage,as hard brittle platelets which form the natural sites in the alloy formechanical weakness. In alloys of the present type, the susceptibilityto sigma phase formation has been found to be directly dependent uponthe concentrations in solid solution of chromium,cobalt, iron,molybdenum and the like.

In the alloy of the present invention, substantial improvements in creepresistance have been obtained without any evidence of sigma phaseprecipitation. Sigma has not, in fact, been observed after engineoperation and exposure of the alloy to 1,040 hours at 1,600F.

A number of alloys were run to a variety of chemistries and varying heattreatments. Each casting was xrayed to determine soundness and freedomfrom inclusions, and macro-etched for grain size and uniformity. Bothpolycrystalline and columnar-grained castings were actually enginetested.

The as-cast microstructure of the polycrystalline FA-375 alloy provedtypical of the rich nickel-base superalloys, i.e., a high volumepercentage of the secondary 7' phase with associated regions of aeutectic 'y-y. Similarly dispersed, though in low volume percent, werecarbides of the MC-type. Gross segregation was not evident.

Initial 'y phase solutioning, as observed by electron microscopy wasevident at approximately 2,100F. and quite apparent by opticalmicroscopy at 2,150F. Alloy homogenization was readily noticeable as theexposure temperature was raised toward that of melting. Incipientmelting and eutectic dissolution began at 2,250F. and no degeneration ofthe primary carbides was observed for any exposure.

The effect of heat treatment on the alloys was pronounced. and, in mostrespects, followed basic correlations between microstructure andmechanical properties. Three basic heat treatment cycles wereinvestigated. The first involved partial 'y' solutioning (2,200F/4hrs.); reprecipitation and 'y' overage (2,000F./4hrs.) and finalprecipitation (l,600F./12 hrs.). The second involved the first and thirdcycles of the above, i.e., 2,200F. solutioning and 1,600F. finalprecipitation. The third utilized only the latter two cycles overagingand final precipitation.

The number three cycle was found to produce optimum levels of strengthand ductility for the alloy when cast into random polycrystalline form.The number two cycle was found best when the alloy was directionallysolidified. Thus the preferred heat treatment cycle for the equiaxedgrain FA-375 alloy contemplates: solution heat treatment at l,950-2,000Fin vacuum of dry TemP- 01% argon or hydrogen for four hours withsubsequent cool- 5 RT 120.200 137.600 6.5 m mg at a rate equ1valent toan cool or faster plus precipi- 5 s tation at l,600F i 25. The preferredheat treatment L200 152300 for the columnar grain FA-375 alloycontemplates: so- 1600 105,800 118,500 6.0 9.0

o o lut1on heat treatment at 2.175 2,200 F for hours 1n 800 58,60075,500 To M same env1ronment as above plus prec1p1tat1on at 10 7 I H m a1,600F i 25.

in testing, no structural change was observed after 890 hours at 1,400F.under a continuously applied ,5 load of 85,000 p.s.i. After 54 hours at1,800F. and Table m 29,000 p.s.i., the only change is phase coalescencealigning in the direction of applied stress. After 1,040 Tens": Raul(Cmumna' Grain) h o l l d f 11.1. 22001=./4 hrs. l600F./l2 hrs.

ours at 1,000 F. under a continuous y app ted 0a 0 Temp 4 01% UTS' Ni40,000 p.s.1., only phase coalescence and anmhilatlon of the fineprecipitate was observed. Had the alloy been RT 133900 1641600 pronetoward sigma formation, this test would have 1300 64,750 79,800 13.732.2 provided opportunity for its presence. However, none was detected.W

' Table 1v Creep-Rupture Test Results (Columnar Grain) Stress Time FinalTemp. "F. p.8.i. to l%,hrs. Life, hrs. Prior EL,% EL,%

V The results of mechanical te stiiig JWe alToTre a 4 m TablEV WM 7summarized in the following tables.

Tensile Test Results Table I Columnar Grain, Carbon less than 200 p.p.m.

Temp. F. 0.2% (S UTS, p.s.i. EL,% RA,%

Creep and Creep Rupture (Equiaxed Grain) Stress Time RT 137,600 160,1009.0 13.8 Alloy Temp. F. p.s.i. to 1% hrs. Life, Hrs. 40

A-375 0 000 74 i A :28 333 $2: 1600 109,600 135,700 12.5 30.2 M375 160050,000 133 338 Alloy A 600 49,600 36 so 1800 78,600 89,900 7.5 28.0FA-375 1700 35,000 82 215 Alloy A 1700 34,300 20 62 2000 37350 491800FA-375 1800 29,000 17 53 Alloy A 1800 28,400 6 40 FA-375 1800 22,000 255Alloy A 1800 21,500 42 T able vl W Creep-Rupture Test Results ColumnarGrain, Carbon below 200 p.p.m. Stress Time Final Temp. F. p.s.i. to 1%,hrs. Life, hrs. Prior EL,% EL,%

Table II Tensile Test Results (Equiaxed Grain) Iii thersw aiigmsilaisummary it will be noted in particular that the creep and creep-rupturestrengths of the alloy of the present invention are vastly superior tothose of Alloy A. As directionally solidified, the FA-S- 75 alloy is thestrongest alloy of this type in its density range. Thus, there has beenprovided a cast, precipitation hardening nick'elbase superalloy havingspecific utility in gas turbine engine blades and vanes, andincorporation of such blades and vanes in production hardware iscurrently in process.

scribed in detail in connection with specific examples and preferredembodiments, the invention in its broader aspects is not limitedthereto, but departures may be made therefrom within the scope of theaccompanying claims without departure from the principles of theinvention and without sacrificing its chief advantages.

' l0 Although the invention has been specifically de- What is claimedis:

long term stability under conditions of high stress at temperatures inexcess of l,650F.

2. "rheiiib'y'dr claim 1 wherein: Y the alloy microstructure as cast ischaracterized by a columnar grained microstructure aligned substantiallyin the direction of solidification.

* i il

2. The alloy of claim 1 wherein: the alloy microstructure as cast ischaracterized by a columnar grained microstructure aligned substantiallyin the direction of solidification.